The inherent assumption on which chroma sub-sampling is founded is that luminance information is far more important than chromanance information. This is backed up by empirical evidence. From there, the effect follows directly from the equation for luminance:
Y = 0.2126 R + 0.7152 G + 0.0722 B
From here you can see that green is by far the most important color, followed by red, and then finally, way down there, blue.
Y = 0.2126 R + 0.7152 G + 0.0722 B
From here you can see that green is by far the most important color, followed by red, and then finally, way down there, blue.